The present invention relates to a radio receiver having a circuit arrangement for checking the quality of reception at an alternative frequency to a received transmit frequency, having two tuners for converting the received signals to different intermediate frequencies and a switch for optionally forwarding signals from signal processing branches that are each connected to one of the tuners.
Modern radio receivers, particularly those used in multiple locations, as is preferably the case with car radios, are equipped with a circuit arrangement for checking the reception quality of a set station at an alternative frequency of this station. The information about alternative frequencies is generally derived from encoded information that is transmitted during a broadcast, for example using RDS (radio data system) signals. The reception quality at one or more alternative frequencies is checked either regularly or when the reception quality of the selected station at the set frequency deteriorates.
Known methods include equipping the radio receiver with two separate receivers for this purpose, with one receiver continuously selecting and checking alternative frequencies in the background. For isolation purposes and to avoid mutual disturbances, the two receivers have different intermediate frequencies. A microprocessor compares the quality of the station received by the background receiver at the alternative frequency with the set program. This background check is carried out without the user noticing any interruption in reception. However, this concept is relatively costly because both receivers must be fully equipped with tuners, IF filters, IF stages with demodulator, RDS decoders, signal quality test equipment, etc.
Another known method is to check the alternative frequencies using only one receiver. To do this, the tuner""s PLL (phase-locked loop) briefly switches to an alternative frequency. Reception at the alternative frequency is checked and a determination is made as to whether or not the alternative frequency provides better reception than the frequency currently set. If reception at the alternative frequency is poorer, the receiver switches back to the original frequency and possibly checks another alternative frequency. Although this concept is more economical, it has the disadvantage that the check pauses and briefly, though noticeably, interrupts the program to which the user is listening.
According to the related art, therefore, a problem arises in that either the check of alternative frequencies can be carried out only at greater expense or, to reduce costs, the user must put up with a noticeable interruption in the program to which he is listening.
An object of the present invention is therefore to avoid the high cost of using two separate and fully equipped receivers and also to at least reduce interference with the received program.
According to the present invention, this object is achieved with a radio receiver of the type mentioned in the preamble by positioning the switch upstream from an analog-digital converter, operating at a sampling frequency (fa), of an intermediate frequency stage; by selecting the different intermediate frequencies so that they both meet the following condition:
f0=(n+R)fa
where n is a natural number, and 0xe2x89xa6Rxe2x89xa61 applies; and an intermediate frequency filter, a demodulator, possibly a decoder for additional signals, and a test unit for determining signal quality are connected to the analog-digital converter.
According to the present invention, two tuners that convert the received signal to different intermediate frequencies are used, like in the concept with two receivers. However, both tuners are provided with a common intermediate frequency stage having an intermediate frequency filter, a demodulator, a decoder for additional signals, and a test unit for the signal quality, so that either the intermediate frequency of the one tuner or the intermediate frequency of the other tuner can be directed to the intermediate frequency stage via the switch, which is designed, for example, as a multiplexer. This is possible because the analog-digital converter operating at a sampling frequency (fa) can digitize the different intermediate frequencies when both intermediate frequencies meet the following condition:
f0=(n+R)fa
as is generally known from German Patent Application No. 41 04 882. This makes it possible, according to the present invention, to process both intermediate frequencies using the same intermediate frequency stage without any additional means.
To check the alternative frequencies, therefore, the switch, which is preferably controlled by a microprocessor, interrupts processing of the signals received by the set tuner, thus checking the alternative frequency. Unlike the concept using one receiver, the interruption needs to be carried out only for the brief check period, while the corresponding PLL must resonate to the new mixed frequency when a single tuner switches over, so that, when switching to an alternative frequency and then back to the original frequency, two transient responses lasting just milliseconds are added to the check time and form the overall interrupt time. The interruption produced according to the present invention is therefore shorter by at least a factor of 2 so that the interruption to the received program is barely noticeably. If the arrangement checks an alternative frequency that is known to have a usable reception quality, due to an earlier check, the interruption to the received program is not noticeable at all.
If, however, there is the possibility that the reception at the alternative frequency is very poor, possibly causing a very noisy signal to be received, it is suitable to mute the low-frequency channel for the brief check period to prevent any disturbing noise signals from passing through to the speaker.
Because the interruption during the check according to the present invention is so short that it does not noticeably disturb the received program, it may be suitable according to the present invention to create and store a displayable table of frequencies, i.e., stations, that can be received at the reception location by checking a multiplicity of frequencies, making it possible to select a specific number of suitable stations, i.e., transmit frequencies.